The precise measurement of very thin films, i.e. films 300 .ANG. thick or less, is extremely important in the semiconductor industry. Such films may be present on a semiconductor by design, or in some cases, they may be there in the form of contamination. In particular, accurate thickness measurement of silicon dioxide films on monocrystalline and polycrystalline silicon in the range from 50 to a few hundred angstroms is very important, as these films are used as gate oxides and floating gate oxides. Likewise, nitrides and oxynitrides of comparable thickness may be used where improved dielectric breakdown strength is needed. As another example, in certain cleaning steps it is required that no residues be left on a wafer. These residues may range from monomolecular layers to layers several hundred angstroms thick. In this case, a precise thickness measurement is not as important as a positive confirmation of their presence. Similar problems in GaAs, GaAlAs and other semiconductors also require thin film measurement.
Reflectivity measurements on silicon and other semiconductors are routinely made in order to measure the thickness of deposited or grown films. Typically, these reflectivity measurements are made in the visible part of the light spectrum. For relatively thick films, maxima and minima are observed in a plot of reflectivity versus wavelength, due to the light interference taking place in the films. The film thickness can be inferred from the wavelength positions of these various extrema. However, if the film is very thin, no minima or maxima may be observed, and an absolute measurement of the reflectivity must be made to determine film thickness. These absolute reflectivity measurements are usually done in the wavelength range from 400 to 500 nm.
Wesson, Phillips and Pliskin in the Journal of Applied Physics, pp. 2455-2460, 1967, describe using an ultraviolet spectrophotometer, with variable wavelength, to measure the film thickness of silicon dioxide several thousand angstroms thick. The location of the minima and maxima were used to determine the thickness. There is, however, relatively little advantage to this approach over the usual visible light measurements, because in this thickness range, the measurements can be done in the visible part of the spectrum with equal accuracy and less difficulty.
Chiang, Dell 'Occa and Schwettmann in the Journal of the Electrochemical Society, pp. 2267-2268, 1979, describe making ultraviolet measurements at variable wavelength to infer the roughness of polysilicon layers.
Duffy et al. in Journal of Crystal Growth, pp. 10-18, 1982 and Harbeke et al. in RCA Review, vol. 44, pp. 19-29, 1983, describe an ultraviolet reflectometer used to measure properties of silicon grown on sapphire or polysilicon. The instrument uses a deuterium source with a selection of two wavelengths (.lambda..sub.1 =280 nm, .lambda..sub.2 =400 nm) and measures the reflectivity at both wavelengths. The measured reflectivity is asserted to be affected by absorption due to degraded crystallinity and surface texture at 280 nm, but only by surface texture at 400 nm. Thus, information can be gained about the structure and quality of the films from these measurements.
An object of the present invention is to provide a more accurate method of measuring the thickness of thin films or substrates.